The present invention relates to a sintered valve seat ring for an internal-combustion engine.
In practice, valve seat rings for internal-combustion engines are made mainly of cast or sintered metal alloys. Due to their excellent resistance to wear, particularly to corrosion and to stresses caused by alternating temperatures during operation of the engine, metal alloys based on nickel or cobalt are primarily used. However, these metals are relatively expensive and their costs will rise with the increasing scarcity of the raw materials.
As a substitute, valve seat rings have been made of highly alloyed types of cast iron or sintered iron. However, such valve seat rings do not have the required high resistance to stresses during engine operation. For that reason, the seating faces of the rings are often protected by surface treatments. When an armor coating is applied by, for example, deposition welding, the adhesion of the coating on the ring must be particularly good since during operation of the engine the seating face is stressed by the impacts of the valve head, possibly causing the coating to chip off.
Other surface treatment processes, such as surface hardening with nitrides or borides, as is customary with other machine parts that are subject to friction, cannot be used with valve seat rings since the rings must be ground before being installed and such grinding would remove the relatively thin, hardened coatings.
Machine parts such as sealing strips or piston rings for internal-combustion engines, have been conventionally sintered with different sintering substances for the supporting portion and the facing portion (that is, the portion subject to wear). This method also presents the problem of providing particularly good adhesion between the portions, because of the high impact stresses imparted by the valve head during engine operation. To overcome this problem, German Pat. No. 2,139,738 discloses sealing strips or gaskets which are sintered of a mixture of a steel alloy powder and a carbide powder in such a way that the proportion of carbide in the supporting portion is considerably less than the proportion of carbide in the portion subject to wear. Since, however, the binder metal is the same in both cases, the machine part can be sintered in a single process step. Advantageously, however, at least the facing portion of valve seat rings should consist of nickel or cobalt alloys which are basically binder metals and which have a lower sintering temperature than steel.